Electrolytic bath for depositing chromium



Patented Au 11, 1936 ELEc'raoLmo BATH FOR. n rosrrmG CHROMI Christian Wiokenhiser, Detroit, Mich.

No Drawing. Application August 10, 1935, Serial No. 35,630

10 Claims.

5 lution of chromic acid with the addition of acid radicals, usually suphates either in the form of chromic sulphate or sulphuric acid. An object of my invention is to eliminate the use of sulphate and in its stead employ a new agent which not only produces a chromium plating of satisactory hardness and density but noticeably increases the throwing power of chromium plating baths.

It is a further object ofmy invention to reduce the cost of electroplating chromium by employing a distinctly new typeof bath which requires in addition to the chromium a very small amount of another substance, 9. preferred proportion being substantially one part of this substance to 50 parts'of chromium compound in the bath. This additional substance is easily and cheaply obtained or, if need be, produced on the premises of the electroplating apparatus. This substance which I have found to be extremely eiilcient in the electrodeposition of chrmium is produced'by mixing boiling solutions of a carbonate salt with a sulphate salt; The precipitate from such solution is the desired product. It is a compound of a metal and hydrous basic carbonate. A typical example of such a compound is magnesium alba (artificial magnesium carbonate) the formula of which is variously written as MgCO3-Mg(OH)a'3H2O or 5MgC03-2Mg(OH)2-7H2O which represent its light form, or as 3MgCOa-Mg(OH)2-4H2O"which representslits heavier form. The magnesium alba as written above consists partly of magnesium hydroxide and partly of magnesium carbonate. After such an alba compound has pre- 40 cipitated, whether of magnesium or any other metal, it is filtered, washed and dried, at which time it is ready for introduction into the bath.

In precipitating compounds of this form for use in the bath, I have found that all the metal sulphate and carbonate compounds, including nickel, cobalt, magnesium, mercury, sodium,;po-,

tassium, lead, cadmium, tin and cerium. Solutions of these sulphate salts have successfully reacted with acarbonate compound, such as sodium carbonate, to produce a precipitate which when introduced into a chromium acid plating bath has materially power. What metal or metals precipitates with the alba radical is apparently of little importance. I

Furthermore, I have found that attempting to introduce the parts of the hydrous basic carbonate compound into the bath separately will increased its throwing v not succeed. That is to say, if an attempt is made to introduce a hydroxide and a carbonate into the bath so that the bath will apparently boiling. At this stage a carbonate salt solutionis introduced into the boiling sulphate solution. A precipitation takes place. This precipitation, as best as can be determined at present, is a hydrous basic carbonate of a metal. If magnesium sulphate is used as the sulphate salt, the precipitation is magnesium alba. This precipitate is filtered, washed and dried. It is now ready for insertion in the bath.

After the hydrous basic carbonate has been formed, it is introduced into a bath of chromic acid. The preferred amount of chromic acid is 2 to 46 ounces for each gallon of water. The preferred proportion of the hydrous basic carbonate compound to the chromic acid is 13/40 ounce of the former compound for each pound of chromic acid. This amounts to approximately one-third of an ounce of hydrous basic carbonate for each pound of chromic acid, orabout one part to 50 .parts respectively. It is-clear that these proportions may be varied to suit different conditions. of chromic acid and a compound containing a hydrous basic carbonate radical.

Of the various metal compounds which may be precipitated as described above, I prefer to use magnesium alba because of its abundance and The bath preferably consists solely I cheapness. Other metals will, however, .work

just as well. Magnesium alba is derived by mixing solutions of magnesium sulphate, and sodium carbonate, boiling, filtering the white precipitate, washing and drying. Other metal compounds containing a hydrous basic carbonate are obtained in a similar manner.

Instead of producing the precipitate by using magnesium sulphate, nickel sulphate, NiSO4, may be used. Chromic sulphate, Crz(SO4)a, may likewise be used in place ofmagnesium sulphate, and the precipitate will contain chromium as an element. Other sulphates of metals may be used in place of magnesium sulphate, including sodium and sodium alum sulphate,

As an illustration of the'variability of preparing the hydrous basic carbonate precipitate several of these sulphate salts may be combined with sodium carbonate in boiling solution and the resultant precipitate used successfully in chromic acid baths. As for example, 1 pound of sodium carbonate solution may be added to a solution containing $4; pound of nickel sulphate, /4. pound of chromium sulphate, /4 pound of magnesium sulphate and A pound of sodium aluminum sulphate. The precipitate derived from mixing these solutions has given very good results. This illustrates that all sulphates, alums, carbonates, sodas and sodium compounds can be combined in different proportions to furnish the precipitate for the chromic acid bath. By this procedure, sulphate is entirely eliminated from the bath and in its place a distinctly new agent is employed.

Instead of precipitating the hydrous basic carbonate compound in a separate or preliminary operation, the compound can be derived in the water which is to form the electrolyte. As for example, magnesium alba could be formed in boiling water by introducing predetermined quantities of magnesium sulphate and sodium carbonate which when they react will precipitate the desired amount of magnesium alba. The sulphate will be reduced by the action and both thesulphate and the sodium will be driven in gaseous form from the boiling water. Instead of filtering the precipitate from the water, chromic acid is added which will dissolve the precipitate and form an electrolytic solution of chromic acid and magnesium alba. This procedure would save the steps of filtering, washing and drying.

After the hydrous basic carbonate compound has been introduced into the chromic acid solution, the plating bath is heated preferably with tin plated steam coils in the bath although hot water jackets may be used if desired. A suitable voltage of from 4 to 6 volts is employed and the current density is kept at from 50 to 125amperes per square foot. The actual voltage and current density depends on the metals which are being plated. -Some metals, howevenrequire 12 volts for a relatively shorttime. of plating. The temperature of the bath is generally maintained between 110 F. and 160 F. I find that baths under these conditions plate at a speed from .001 to .005 of an inch per hour on steel or iron, and are very active upon other metals as well. Baths of this type can be used as a deplater to remove chrome plate. They also clean the metal before the chromium is plated thereon.

My chromium plating bath including the hydrous basic carbonate agent may also include other metals, such as copper, for electroplating an alloy of chromium. A copper chromium alloy may be formed by providing a copper anode in the bath. Other metal anodes have been found to improve my bath, as for example, anodes of lead, tin and aluminum.

What I claim:

1. A chromium electroplating bath including a chromium compound and a compound of a. hydrous basic carbonate.

2. An electroplating bath for electrodepositing chromium which consists solely of water, chromic acid and a compound of a hydrous basic carbonate.

3. An electroplating bath for electrodepositing chromium which consists of water, chromic acid and substantially one-third ounce of a compound of hydrous basic carbonate for each pound of chromic acid.

4. The process of improving the throwing power of a chromium plating bath which comprises adding to a bath of chromic acid a small percentage of a compound having both a. carbonate and a hydroxide radical joined in chemical union.

5. The process of improving the throwing power of a chromium plating bath which comprises preparing a bath of chromic acid and adding thereto a compound containing a hydrous basic carbonate in the relative proportions of substantially one-third once of compound for each pound of chromic acid in the bath.

6. An electroplating bath for electrodepositing chromium which includes water, chromic acid and magnesium alba.

7. An electroplating bath for electrodepositing chromium including for each gallon of electrolyte the following substances in substantially the proportions named; 2 to 46 ounces of chromic acid, and A; ounce of a hydrous basic carbonate compound for each pound of chromic acid.

8. A chromium electroplating bath including in substantially the proportions named; one part of a compound having a carbonate and a hydroxide radical joined in chemical union for each 50 parts of chromic acid in the bath.

9. The process of preparing an electroplating bath for depositing chromium which comprises preparing an addition agent in the electrolyte by reacting carbonate and sulphate compounds in w the electrolyte to form a hydrous basic carbonate compound therein, and subsequently adding chromic acid to the electrolyte.

10. In a chromium electroplating bath, an addition agent for increasing the throwing power of the bath comprising a hydrous basic carbonate compound.

CHRISTIAN WICKENHISER. 

